Recently, organic electroluminescence (EL) displays using organic electroluminescence have been attracting attention as one of next-generation flat panel displays to replace the liquid crystal display. In an active-matrix display apparatus such as an organic EL display, a thin-film semiconductor device referred to as a thin-film transistor (TFT) is used.
A bottom-gate thin-film semiconductor device in which a gate electrode is formed closer to a substrate than to a semiconductor layer is generally used as the thin-film semiconductor device (see Patent Literatures (PTLs) 1 to 3). FIG. 13 is a cross-sectional view showing a structure of a conventional bottom-gate thin-film semiconductor device. In a thin-film semiconductor device 500 shown in the figure, a gate electrode 52 is formed above a substrate 51, and a gate insulating film 53 is further formed above the gate electrode 52 and above the substrate 51 extending beyond the both ends of the gate electrode 52. A semiconductor layer 54 having a channel region is formed above the gate insulating film 53. The semiconductor layer 54 is a polycrystalline silicon thin film formed by crystallizing noncrystalline silicon (amorphous silicon). A source electrode 57S and a drain electrode 57D are formed above the semiconductor layer 54, with a pair of contact layers 56 therebetween. In addition, a channel protection layer 55 servicing as a channel etching stopper layer is formed above the channel region of the semiconductor layer 54.
In a technique of manufacturing the thin-film semiconductor device 500, when the gate insulating film 53 is formed by plasma CVD
(Chemical Vapor Deposition) or the like, a step portion 58 is formed in each of parts of the gate insulating film 53 corresponding to the both ends of the gate electrode 52, due to a film thickness of the gate electrode 52. FIG. 14 is an electron microscopic picture showing a cross-section of the step portion of the gate insulating film in the conventional thin-film semiconductor device. As shown in FIG. 14, a flat portion 59 of the gate insulating film 53 has a film thickness t1 of approximately 110 nm, the gate electrode has a film thickness t2 of approximately 50 nm, and a film thickness ratio R is R=t1/t2=110 nm/50 nm=2.2. The step portion 58 of the gate insulating film 53 has a film thickness t3 of approximately 70 nm, and the film thickness t3 is less than the film thickness t1 of the flat portion 59 of the gate insulating film 53.